The pair interface simulates a normal
Ethernet interface by encapsulating standard network frames with an Ethernet
header, specifically for use in a pair of interfaces that are interconnected
with each other.

To use it, the administrator needs to create two
pair interfaces and connect them; the
interfaces are ‘patched’, as would be done with physical network
ports. All packets that are sent on the first interface are received on the
second interface.

When adding multiple pair to multiple
bridge(4) interfaces, it is
possible to create a loop; the system load will go up while it is busy sending
packets from one bridge to another and back. By design, the driver does not
prevent such loops by itself, but it is possible to use the Spanning Tree
Protocol (STP) to detect and remove loops in the virtual network topology:

Unlike vether(4), the
pair interface cannot be used as a
stand-alone member in a
bridge(4): the link state
remains down until it is connected to the second interface. Any associated
routes will be marked down until it is patched. Use
vether(4) as a bridge endpoint
for routing purposes instead.